Nitroaromatic compositions have been widely used as intermediates in the chemical industry, and are well suited for producing a variety of industrial chemicals such as aromatic amines for use as chain extenders in polyurethane manufacture or in preparation of aromatic isocyanates for urethane synthesis. Examples of widely used nitroaromatic compositions include mono and dinitrobenzene, mono and dinitrotoluene and mono and dinitroxylene.
Numerous processes have been developed for effecting nitration of aromatic hydrocarbons. The conventional procedure for nitrating hydrocarbons has been the mixed acid process wherein concentrated nitric acid and concentrated sulfuric acid are used as the nitrating medium. This process is referred to as the mixed acid process and has been preferred over other nitration procedures since nitration can proceed at a more effective rate due to increased nitronium ion concentration in the nitration medium. Subsequent to nitration, the nitroaromatic is separated from the spent acid phase and further nitration effected, if desired, by recycling the mono or dinitrated aromatic to a subsequent nitration zone. Representative patents which disclose mixed acid techniques include U.S. Pat. Nos. 2,849,497; 3,185,738; and 4,123,466. The U.S. Pat. No. 4,123,466 differs from the other two patents in that nitrogen dioxide is injected into the reaction medium of aromatic compound and sulfuric acid as opposed to nitric acid injection into the reaction medium.
Numerous processes have also been developed which utilize nitric acid alone as the nitrating agent in the manufacture of both mono and dinitroaromatic compounds. Typically, when the dinitro compound is prepared, a more concentrated nitric acid is utilized, e.g. fuming nitric acid as the nitrating agent than when the mononitroaromatic is prepared. Representative patents which disclose various nitration processes using nitric acid along include U.S. Pat. Nos. 2,362,743; 2,739,174; 3,780,116; and 4,112,005. Although the processes employ nitric acid as the nitration medium, they differ in the selectivity to either mono or dinitrated product produced or in the use of different mole ratios of nitric acid to aromatic compounds or in the use of different process techniques to effect nitration.
Another form of nitration procedure for producing nitrated aromatic composition resides in the use of nitrogen oxides alone or in combination with sulfuric acid or oxygen as the nitrating medium. Various nitrogen oxides such as N.sub.2 O.sub.5, NO.sub.2 and N.sub.2 O.sub.4 have been utilized as the nitrating agent. The U.S. Pat. No. 4,028,425 provides for the introduction of nitrogen dioxide and oxygen into an aromatic hydrocarbon with the coproduction of the dinitroaromatic compound and concentrated nitric acid as a product.
Each of the processes described above requires separation of the nitroaromatic compound from the unreacted aromatic compound and spent nitric acid. Typically, the spent nitric acid contains a substantial amount of water which must be removed in order to recover the spent nitric acid and recovering the unreacted aromatic hydrocarbon. Recovery of residual nitric acid in the reaction product is a major commercial object in preparing dinitroaromatic compounds where the amount of residual acid is substantial. In mononitration reactions the amount of residual nitric acid may be less than in dinitration reactions, and recovery may not be as important as is the separation of the nitroaromatic and unreacted aromatic compounds from the spent acid.
Various techniques have been developed to recover nitric acid from the spent acid in a nitration process. For example, U.S. Pat. No. 4,257,986 discloses the separation of the organic phase from the spent acid phase in a mixed acid nitration process and then introducing unreacted aromatic compound into the spent acid phase under nitrating conditions to generate mononitroaromatic compound. The reaction product then is allowed to separate into various phases and the aqueous phase removed from the organic phase containing the nitroaromatic compound and unreacted aromatic.
U.S. Pat. No. 2,849,497 discloses the separation of the organic layer from the aqueous layer which consists primarily of nitric acid, sulfuric acid, nitroaromatic compound and water. The U.S. Pat. No. 2,849,497 process involves the introduction of additional unreacted aromatic compounds into the spent acid phase to denitrate the spent acid. The advantage of the process is that the nitric acid, which is dissolved in the crude nitroaromatic, is advantageously recovered for the preparation of additional product rather than being washed out first.
One of the more conventional techniques for removing spent nitric acid from the reaction mixture, with either the mixed acid process, the nitric acid process or a nitrogen oxides process, is the neutralization of the spent acid by reaction with alkali and the products thus separated. For example, U.S. Pat. No. 1,325,168 discloses the neutralization of the nitric acid with a base, such as lime, ammonia, or the like, and then removing the nitrobenzene in a pure state by distillation. Solvent extraction using carbon tetrachloride is also suggested as being an alternative to the neutralization technique.
U.S. Pat. No. 2,736,742 discloses the uses of a mixture or solution of ammonia and organic bases with ammonia nitrate to effect removal of nitric acid.
U.S. Pat. No. 4,261,908 discloses the separation of dinitroaromatics from nitric acid by distillation.
The separation of nitric acid from nitroaromatic compositions produced in aromatic dinitration processes has been a continuing problem. The problem is more severe in dinitration reactions than in mononitration reactions and is more severe in those processes using nitric acid alone as the nitrating medium. Several reasons are apparent. First, the spent nitric acid present in a dinitration reaction has a much higher concentration than normally used when a mixed acid process is used. Second, removal of the nitric acid typically cannot be effected by the introduction of fresh aromatic because such conditions are not suited for nitration. On the other hand, modest nitration can be achieved by the mixed acid process to the mononitro product. Thirdly, the processes which rely on neutralization techniques in an effort to recover nitrated aromatic compounds from the nitration wash liquors as represented in the U.S. Pat. No. 4,241,229 disclose problems with respect to the sewering and problems associated with biological treatment.